First of all, a synchronization channel (hereinafter abbreviated SCH) as an example of a specific channel is explained as follows.
In order for a user equipment to communicate with a base station in a mobile communication system, the user equipment primarily performs synchronization with the base station on SCH and then secondarily performs a cell search. A series of process for performing synchronization with a base station and obtaining an ID of a cell to which a user equipment belongs is called a cell search.
Generally, a cell search is classified into an initial cell search executed in case of a user equipment's ‘power on’ in an initial mode and a neighbor cell search performed by a user equipment in a connection mode (i.e., normal mode) or idle mode to search a neighbor base station.
FIG. 1 is a flowchart of a cell search procedure.
If the 3GPP LTE system is taken as an example among various communication systems, the SCH structure for the cell search is classified into a hierarchical structure and a non-hierarchical structure according to a time symbol synchronization and cell search method.
And, a cell ID obtaining method is classified into a first method of searching cell groups and searching reference signals for a final cell ID and a second method of obtaining a cell ID from SCH only.
In case of the first method, a cell group ID is obtained from SCH. In case of the second method, a final cell ID (i.e., cell ID) is obtained from SCH.
SCH following the hierarchical structure is explained as follows.
First of all, the SCH following the hierarchical structure is classified into a primary SCH (hereinafter abbreviated P-SCH) and a secondary SCH (S-SCH) like SCH of WCDMA.
The P-SCH is a channel on which all cells or sectors use the same signal and performs initial symbol synchronization and frequency synchronization. A signal value of P-SCH is already known by all user equipments. So, initial time symbol synchronization can be executed by performing cross-correlation between a received signal and a predetermined signal. Such a series of procedures are called cross-correlation based detection.
After time, symbol and frequency synchronizations have been obtained, cell ID or cell group ID detection is performed at a position of S-SCH previously indicated by timing synchronization information obtained from P-SCH.
Meanwhile, a method of multiplexing P-SCH and S-SCH can be classified into TDM, FDM and CDM.
FIG. 2 is a structural block diagram of SCH following a hierarchical structure.
An example shown in FIG. 2 corresponds to an example of multiplexing by TDM. Positions of P-SCH and S-SCH and a count of OFDM symbols including P-SCH and S-SCH may differ from the case shown in FIG. 2.
Referring to FIG. 2, SCH is generated via two OFDM symbols. A radio frame includes 20 sub frames, and a specific radio frame is selected to transmit SCH only.
In the example shown in FIG. 2, first and second OFDM symbol provide SCH. Yet, an OFDM symbol which provides SCH can be another OFDM symbol, e.g., a last OFDM symbol.
SCH following the non-hierarchical structure is explained as follows.
First of all, a non-hierarchical SCH is characterized in having a repetitive waveform within an OFDM symbol in time domain. This enables a user equipment to perform a blind detection of initial time symbol synchronization through auto-correlation of a received signal using the repetitive characteristic of signal. And, this detection is called auto-correlation based detection.
After completion of the time and frequency synchronizations, cell ID detection or cell group ID detection are carried out at a position of the detected SCH.
FIG. 3 is a structural block diagram of SCH following a non-hierarchical structure. An OFDM symbol or subframe to form SCH can be freely changed.
In the above-explained hierarchical structure, cross-correlation based detection can be carried out based on Formula 1.
                                              ⁢                                            d              ^                        =                                          argmax                d                            ⁢                              {                                                      R                    ⁡                                          (                      d                      )                                                        |                                      0                    ≤                    d                    ≤                                                                  N                        f                                            -                      1                                                                      }                                              ⁢                                          ⁢                                    R              ⁡                              (                d                )                                      =                                          ∑                                  p                  =                  1                                P                            ⁢                                                          ⁢                                                ∑                                      q                    =                    1                                    Q                                ⁢                                                                  ⁢                                  (                                                            (                                                                        ∑                                                      m                            =                            1                                                    M                                                ⁢                                                                                                  ⁢                                                                                                                                                                        ∑                                                                  n                                  =                                                                                                            (                                                                              m                                        -                                        1                                                                            )                                                                        ⁢                                    L                                                                                                                                    mL                                  -                                  1                                                                                            ⁢                                                                                                                          ⁢                                                                                                                                    S                                    *                                                                    ⁡                                                                      (                                    n                                    )                                                                                                  ⁢                                                                                                      r                                                                          p                                      ,                                      q                                                                                                        ⁡                                                                      (                                                                          n                                      +                                      d                                                                        )                                                                                                                                                                                                            2                                                                    )                                        /                                          (                                                                        ∑                                                      n                            =                            0                                                                                N                            -                            1                                                                          ⁢                                                                                                  ⁢                                                                                                                                                                        r                                                                  p                                  ,                                  q                                                                                            ⁡                                                              (                                                                  n                                  +                                  d                                                                )                                                                                                                                          2                                                                    )                                                        )                                                              ⁢                                          ⁢                                          ⁢                      (                          N              =              ML                        )                          ⁢                                                      [                  Formula          ⁢                                          ⁢          1                ]            
In Formula 1, ‘R(d)’ is a cost function to find a start point for synchronization acquisition, ‘{circumflex over (d)}’ is a value to maximize the R(d), and ‘Nf’ is a length of a radio frame. ‘P’ indicates a count of symbols used for averaging. ‘Q’ indicates a count of receiving antennas of a user equipment. ‘L’ indicates a count of Parts for M-partial correlation. ‘N’ indicates a point of FFT/DFT operation. ‘rp,q(n)’ indicates a signal received by a qth receiving antenna in a pth P-SCH symbol. And, ‘S(n)’ indicates a known sequence inserted in P-SCH. In this case, in the environment where frequency offset exists, if symbol synchronization via cross-correlation based detection is simply carried out, performance is degraded. So, M-partial correlation can be applied. [Y.-P. E. Wang and T. Ottosson, “Cell search in W-CDMA”, Selected Areas in Communications, IEEE Journal on, vol. 18, pp. 1470-1482, August 2000.]
In the hierarchical structure shown in FIG. 2, a method of estimating frequency offset can be represented as Formula 2.
                              Δ          ⁢                                          ⁢          f                =                                            f              s                                      π              ⁢                                                          ⁢              N                                ⁢          arg          ⁢                      {                          -                                                ∑                                      p                    =                    1                                    P                                ⁢                                                                  ⁢                                                      ∑                                          q                      =                      1                                        Q                                    ⁢                                                                          ⁢                                                                                    [                                                                              ∑                                                          n                              =                              0                                                                                                                      N                                /                                2                                                            -                              1                                                                                ⁢                                                                                                          ⁢                                                      (                                                                                                                            S                                  *                                                                ⁡                                                                  (                                  n                                  )                                                                                            ⁢                                                                                                r                                                                      p                                    ,                                    q                                                                                                  ⁡                                                                  (                                                                      n                                    +                                                                          d                                      ^                                                                                                        )                                                                                                                      )                                                                          ]                                            *                                        ⁢                                                                                         [                                                                              ∑                                                          n                              =                                                              N                                /                                2                                                                                                                    N                              -                              1                                                                                ⁢                                                                                                          ⁢                                                      (                                                                                                                            S                                  *                                                                ⁡                                                                  (                                  n                                  )                                                                                            ⁢                                                                                                r                                                                      p                                    ,                                    q                                                                                                  ⁡                                                                  (                                                                      n                                    +                                                                          d                                      ^                                                                                                        )                                                                                                                      )                                                                          ]                                            }                                                                                                                              [                  Formula          ⁢                                          ⁢          2                ]            
In Formula 2, ‘fs’ means a sampling frequency and ‘arg{ }’ means a phase component for a complex number. And, the frequency offset is generated from the difference between frequencies generated by oscillators provided to a base station and a user equipment, respectively.
And, the auto-correlation based detection used for the non-hierarchical structure shown in FIG. 3 is represented as Formula 3.
                              R          ⁡                      (            d            )                          =                              ∑                          p              =              1                        P                    ⁢                                          ⁢                                    ∑                              q                =                1                            Q                        ⁢                                                  ⁢                          (                                                                                                                                      ∑                                                  n                          =                          0                                                                                                      N                            /                            2                                                    -                          1                                                                    ⁢                                                                                          ⁢                                                                                                    r                                                          p                              ,                              q                                                        *                                                    ⁡                                                      (                                                          n                              +                              d                                                        )                                                                          ⁢                                                                              r                                                          p                              ,                              q                                                                                ⁡                                                      (                                                          n                              +                              d                              +                                                              N                                /                                2                                                                                      )                                                                                                                                                    2                                /                                                      ∑                                          n                      =                      0                                                                                      N                        /                        2                                            -                      1                                                        ⁢                                                                          ⁢                                      (                                                                                                                                                                                r                                                              p                                ,                                q                                                                                      ⁡                                                          (                                                              n                                +                                d                                                            )                                                                                                                                2                                            +                                                                                                                                                            r                                                              p                                ,                                q                                                                                      ⁡                                                          (                                                              n                                +                                d                                +                                                                  N                                  /                                  2                                                                                            )                                                                                                                                2                                                              )                                                              )                                                          [                  Formula          ⁢                                          ⁢          3                ]            
And, the frequency offset can be estimated in the non-hierarchical structure in a following meaner of Formula 4.
                              Δ          ⁢                                          ⁢          f                =                                            f              s                                      π              ⁢                                                          ⁢              N                                ⁢          arg          ⁢                      {                          -                                                ∑                                      p                    =                    1                                    P                                ⁢                                                                  ⁢                                                      ∑                                          q                      =                      1                                        Q                                    ⁢                                                                          ⁢                                                            ∑                                              n                        =                        0                                                                                              N                          /                          2                                                -                        1                                                              ⁢                                                                                  ⁢                                                                                            [                                                                                    r                                                              p                                ,                                q                                                                                      ⁡                                                          (                                                              n                                +                                                                  d                                  ^                                                                                            )                                                                                ]                                                *                                            ⁡                                              [                                                                              r                                                          p                              ,                              q                                                                                ⁡                                                      (                                                          n                              +                                                              d                                ^                                                            +                                                              N                                /                                2                                                                                      )                                                                          ]                                                                                                                  }                                              [                  Formula          ⁢                                          ⁢          4                ]            
The cell search method uses the same method for both of the hierarchical structure and the non-hierarchical structure.
Comparison between the hierarchical SCH and the non-hierarchical SCH is explained as follows.
First of all, communication systems are classified into a synchronous network and an asynchronous network. In particular, the synchronous network is a network having the same transmission start time for all sectors. Yet, in the asynchronous network, although sectors within one Node-B have the same transmission start time, a transmission start time is random between Node-Bs.
In case that both of the synchronous network and the asynchronous network need to be supported, the hierarchical SCH is preferably used.
FIG. 4 is a diagram of performance comparison between hierarchical SCH and non-hierarchical SCH in a synchronous system. And, FIG. 5 is a diagram of performance comparison between hierarchical SCH and non-hierarchical SCH in an asynchronous system.
Referring to FIG. 4 and FIG. 5, a case of using a hierarchical structure improves performance of cell search. Yet, in the hierarchical structure, cross-correlation detection should be used for timing synchronization acquisition and frequency offset estimation performance of the hierarchical structure is inferior to that of a non-hierarchical structure. FIG. 6 is a diagram of residual frequency offset error in a synchronous system, and FIG. 7 is a diagram of residual frequency offset error in an asynchronous system.
In brief, each of the hierarchical SCH and the non-hierarchical SCH has its own problems.
To solve these problems, the hybrid SCH scheme has been proposed. In the hybrid SCH scheme the hierarchical SCH and the non-hierarchical SCH are combined together.
FIG. 8 is a diagram to explain a hybrid SCH scheme.
Referring to FIG. 8, a hybrid SCH includes P-SCH and S-SCH like the hierarchical SCH. Yet, P-SCH is assigned to a frequency index with a specific interval in a specific OFDM symbol. In other words, a sequence included in P-SCH is configured in a manner that a specific waveform is repeated in time domain by a prescribed count.
Meanwhile, S-SCH is configured in a same manner of the hierarchical SCH.
In case of the hybrid SCH, P-SCH is configured by a cell common sequence. Yet, by allocating the respective sequences (P0, P1, P2, . . . , PN-2, PN-1) corresponding to P-SCH to a frequency domain with a specific interval, cell search and frequency offset characteristics are enhanced.
Formula 5A shows a method of estimating a synchronization in a hybrid SCH and Formula 5B shows a method of estimating a frequency offset.
                                              ⁢                                            d              ^                        =                                          argmax                d                            ⁢                              {                                                      R                    ⁡                                          (                      d                      )                                                        |                                      0                    ≤                    d                    ≤                                                                  N                        f                                            -                      1                                                                      }                                              ⁢                                          ⁢                                    R              ⁡                              (                d                )                                      =                                                            (                                                            ∑                                              m                        =                        0                                                                    M                        -                        1                                                              ⁢                                                                                  ⁢                                                                                                                                                ∑                                                          n                              =                              mL                                                                                                                                                        (                                                                      m                                    +                                    1                                                                    )                                                                ⁢                                L                                                            -                              1                                                                                ⁢                                                                                                          ⁢                                                                                                                    p                                *                                                            ⁡                                                              (                                n                                )                                                                                      ⁢                                                          r                              ⁡                                                              (                                                                  d                                  +                                  n                                                                )                                                                                                                                                                                2                                                        )                                /                                  (                                                            ∑                                              n                        =                        0                                                                                                                          N                            fft                                                    /                          2                                                -                        1                                                              ⁢                                                                                  ⁢                                                                                                                    r                          ⁡                                                      (                                                          d                              +                              n                                                        )                                                                                                                      2                                                        )                                            +                                                (                                                            ∑                                              m                        =                        0                                                                    M                        -                        1                                                              ⁢                                                                                  ⁢                                                                                                                                                ∑                                                          n                              =                              mL                                                                                                                                                        (                                                                      m                                    +                                    1                                                                    )                                                                ⁢                                L                                                            -                              1                                                                                ⁢                                                                                                          ⁢                                                                                                                    p                                *                                                            ⁡                                                              (                                                                                                                                            N                                      fft                                                                        2                                                                    +                                  n                                                                )                                                                                      ⁢                                                          r                              ⁡                                                              (                                                                                                                                            N                                      fft                                                                        2                                                                    +                                  d                                  +                                  n                                                                )                                                                                                                                                                                2                                                        )                                /                                  (                                                            ∑                                              n                        =                        0                                                                                                                          N                            fft                                                    /                          2                                                -                        1                                                              ⁢                                                                                  ⁢                                                                                                                    r                          ⁡                                                      (                                                                                                                            N                                  fft                                                                2                                                            +                              d                              +                              n                                                        )                                                                                                                      2                                                        )                                                              ⁢                                          ⁢                                          ⁢                      (                                                            N                  fft                                /                2                            =              ML                        )                                              [                  Formula          ⁢                                          ⁢          5          ⁢          A                ]                                          f          ^                =                                            N              fft                                      2              ⁢              π              ⁢                                                          ⁢                                                N                  fft                                /                                  N                  rep                                                              ⁢          arg          ⁢                      {                                          ∑                                  n                  =                  0                                                                                            N                      fft                                        /                    2                                    -                  1                                            ⁢                                                          ⁢                                                                    [                                          r                      ⁡                                              (                                                                              d                            ^                                                    +                          n                                                )                                                              ]                                    *                                ⁡                                  [                                      r                    ⁡                                          (                                                                        d                          ^                                                +                        n                        +                                                                              N                            fft                                                    /                          2                                                                    )                                                        ]                                                      }                                              [                  Formula          ⁢                                          ⁢          5          ⁢          B                ]            